Atomic oxygen protective coatings typically include SiO.sub.x (where x is between 1.9 and 2.0), fluoropolymer-filled silicon dioxide films, and Al.sub.2 O.sub.3. These coatings may be deposited as thin films by ion beam sputtering, electron beam evaporation, chemical vapor deposition, RF magnetron deposition, reactive DC magnetron sputter deposition and the like. Protective coatings for atomic oxygen have been applied over polyimides, graphite epoxy, fluorinated ethylene propylene, and various forms of carbon (such as pyrolytic graphite and carbon-carbon composites).
Abrasion and flexure however, of these protective coatings lead to defects which are susceptible to oxidation. In addition, defects occur as a result of particulate contaminants contacting the surface of the organic material; microscopic roughness and undulations on the protective coating surface; and micrometeoroids or other debris which impact on the protective coating in low earth orbit. These defects enable atomic oxygen to react with the underlying organic, polymeric, substrate.
Atomic oxygen gradually undercuts at defect sites in protective coatings thus oxidizing the underlying polymer. A cavity is formed with portions of the protective coating overhanging this cavity.
For typical spacecraft materials such as polyimide, only about 14% of the incident atomic oxygen will react with the polymer at the bottom of each defect site. Most of the unreacted atomic oxygen that leaves the organic surface is ejected out of the defect opening and back into space. However, as the undercut cavity grows larger around each defect, more and more atomic oxygen, which does not react upon first impact, scatters off the bottom of the atomic oxygen shielding layer. As the atomic oxygen scatters, it again has an opportunity to react with the polymeric material.
U.S. Pat. Nos. 4,560,577, 4,604,181 (Mirtich et al.) and 4,664,980 (Sovey et al.) disclose coating a polymeric substrate with metal oxide to provide protection from oxidation in low earth orbits. Such metal oxide coatings are, however, subject to undercut cavities caused by atomic oxygen.
Protective coatings such as the coating disclosed in the '577, '181 and '980 patents do not recombine atomic oxygen. Consequently, the undercut cavities which inevitably form are continuously exposed to atomic oxygen, which results in a dramatic reduction in the functional lifetime of the underlying structure. Unchecked undercutting ultimately leads to structural failure of protected organic substrates. In the case of solar arrays, this could lead to limiting the duration of the mission or the useful life of the solar array.
U.S. Pat. No. 3,682,100 (Lindberg, Jr.) discloses using a heat-dissociable material for layers which alternate with metal layers in a space vehicle nose cone. The heat dissociable material is selected from hydrides and silver oxide. The outer layer is preferably constructed of ceramic so that upon dissociation of silver oxide, the oxygen does not oxidize the carrier material, but rises to the surface of the nose cone.
U.S. Pat. No. 3,378,411 (Bergen) discloses inhibiting stress corrosion cracking in austenitic stainless steel, particularly upon exposure to chloride, by coating the steel surfaces with an oxide film containing silver, lead and/or cobalt ions.
U.S. Pat. No. 4,757,512 (Macken) discloses silver oxide as a catalyst for oxidizing carbon monoxide to carbon dioxide. There is however, no suggestion of protective coatings for aircraft.